Aluminum alloy, particularly in sheet form, is extremely useful because of its lightness in weight, proper strength, high corrosion resistance and easiness of forming. Paint coating processes are increasingly applied to various kinds of products of aluminum alloy sheet instead of the anodizing process which improves corrosion resistance. For example, cans, caps, and blinds fabricated from aluminum alloy sheet are preferably coated with paint which is baked or cured at high temperatures. Recently, however, such application is extending to those products which require high strength such as body plates or some structural members of buses, rolling-stock and motor cars. For aluminum alloy sheets to be formed by deep drawing, bending or press-forming, the Al-Mn-Mg or Al-Mg alloy of work-hardening type, such as AA-3004, AA-5052 or AA-5082, has been used up to this time. These alloy sheets are, however, susceptible to stretcher-strain marks on the worked or formed surfaces. The Al-Cu-Mg alloy, such as AU2G or X2036, which was developed for manufacturing car-body plates, is undesirable because its strength may be adversely effected by baking of coated surfaces. In such a case, therefore, the Al-Cu, Al-Mg-Si or Al-Zn-Mg of age-hardening type, such as AA-2017, AA-6061, AA-6151 or AA-7075 is recommended because the strength won't be adversely effected by the high temperature of baking, while such shortcomings as described below are on the other hand unavoidable to the latter:
1. The alloy of age-hardening type needs heat-treatment equipment, such as a salt-bath which makes it difficult to handle coiled material.
2. The to corrosion resistance Al-Cu or Al-Zn-Mg-Cu alloy of age-hardening type is inferior to that of work-hardening type in general.
3. Al-Mg-Si alloy is poor in strength and bulging properties.
4. Strength of Al-Zn-Mg alloy which is favored with relatively high corrosion resistance is comparable with that of hitherto used alloy, such as AA-3004, AA-5082 or AA-5083.